Silver halide grains which have two or more twin planes parallel to each other within one grain and take tabular shapes (hereinafter referred to as "tabular grains") are endowed with various advantageous photographic characteristics.
Further, in general, silver halide grains which are used in photographic materials are desirably produced only in a uniform grain shape and grain size. This is because if grains having different shapes are mixed or grain size distribution is broadened (narrow grain size distribution is hereinafter referred to as "high in monodispersibility"), there generate drawbacks such that all the grains cannot be optimally sensitized when the grains are subjected to chemical sensitization or spectral sensitization, or an interlayer effect cannot be put to practical use. With respect to production methods of tabular grains, various technical discussions have been done to improve these drawbacks. Examples thereof are disclosed in, for example, JP-A-63-151618 (the term "JP-A" as used herein means an "unexamined published Japanese patent application" ), JP-A-63-11928, JP-A-2-28638, JP-A-1-131541, JP-A-2-838, JP-A-2-298935, JP-A-3-121445, U.S. Pat. Nos. 4,439,520 and 4,433,048.
Conventional production methods for obtaining tabular grains comprise nucleation, Ostwald ripening and grain growing processes.
Nucleation is a process for generating a new silver halide crystal nucleus (hereinafter merely referred to as "a nucleus") by supplying an aqueous solution of silver salt and an aqueous solution of halide in the presence of a substance capable of becoming a protective colloid such as gelatin.
It is ideal that only the nuclei having parallel twin planes (hereinafter referred to as "tabular nuclei") are formed during the nucleus generating process, but as it is not possible, grains other than tabular nuclei have to be vanished in Ostwald ripening process (hereinafter referred to as "ripening"). Thereafter, tabular grains are grown to obtain the objective grains by again supplying a solution of water-soluble silver salt and a solution of water-soluble halide in the grain growing process.
For example, the formation of monodisperse tabular grains by forming nuclei under the conditions of the temperature and pBr of low solubility in the nucleation process is discussed in recent years.
As is disclosed in JP-W-5-500122 (corresponding to U.S. Pat. No. 4,797,354) (the term "JP-W" as used herein means an "unexamined published International patent application based on a Japanese patent application"), they succeeded in the formation of monodisperse tabular grains having narrow distance between twin planes by setting pBr to reach low solubility of silver halide and forming nuclei using oxidized gelatin.
However, according to conventional production methods, as ripening is carried out immediately after nucleation for forming monodisperse tabular grains, a considerable number of tabular nuclei formed in the nucleation process are dissolved during ripening. Accordingly, the absolute number of tabular grains is lessened and the size of the tabular grain obtained after ripening becomes large.
From the above, it has been difficult to produce tabular grains of a small size and high monodispersibility by conventional techniques.
A method for producing tabular grains which has conventionally been discussed so far to resolve this problem is described below.
JP-A-63-151618 (corresponding to U.S. Pat. Nos. 5,219,720 and 5,334,495) discloses a method of selectively growing tabular grains in low supersaturation in certain concentration of gelatin and pBr value in the latter stage of the ripening process.
However, according to the above method, the first nucleation is conducted at pBr value in the range of from 1.0 to 1.3. After ripening, the pBr is adjusted to a higher condition by the addition of an aqueous solution of silver nitrate and ripening is further progressed. On the contrary, the method of the present invention comprises adding an aqueous solution of silver salt or an aqueous solution of halide within the period of time before ripening progresses after the first nucleation, therefore, the method of the present invention is substantially different from the above method.